.8^. OBSERVATIONS ON THE BRACHIOPODA. 609 



The wide distribution in the North and South Atlantic and the 

 South Pacific Oceans of this geratologous type renders it additionally 

 interesting. It is evident that it is not always an easy matter to 

 distinguish between the youthful and senile brachiopod, for the 

 Brachiopodist meets with the same difficulty as the philologist 

 treating of the Chinese language, namely, that of determining 

 whether it is in its first or second childhood. 



While fully admitting certain resemblances between the 

 typembryo of the brachiopod and the larval stages of Spirorbis, Mr. 

 Beecher refers to the existence of several important differences. He 

 shares the opinion expressed by Joubin, CEhlert, Balfour, and other 

 recent investigators, that the Brachiopoda constitute a distinct and 

 independent class. There seems to be a general reaction in favour 

 of this view, consistently upheld throughout by Dall and Davidson. 

 King was the first to express the opinion that the articulated Clisten- 

 terata were the degenerated descendants of their aniferous prede- 

 cessors, the Tretenterata. But it has always been difficult to account 

 for the presence of an anal opening in the hingeless forms, and its 

 absence in the articulated genera. In this connection the facts are 

 justly emphasised by Mr. Beecher, that the dorsal beaks oi AvipJiigenia, 

 Athyyis, Atvypa, and RhynchoneUa are usually notched, or perforate from 

 the union of the crural plates above the floor of the beak, leaving a 

 passage through to the apex, and that a similar opening occurs between 

 the cardinal processes in StropJiomena, Strophcodonta, and allied genera 

 with a furrowed deltidium, as in Leptana. " This character, which can- 

 not be considered as in any way connected with the pedicle opening (9), 

 points to the existence in the early articulate genera of an anal 

 opening dorsal to the median line," a character which Dr. Joubin first 

 demonstrated is peculiar to Crania (3, 5) among the recent species. 

 This, the " dorsal foramen " of King and Hall, is also known as a 

 " visceral foramen." Mr. Qihlert has suggested that it was probably 

 occupied by the terminal portion of the intestine. 



Mr. Beecher considers the group hitherto known as Arthropomata 

 (Owen) or Clistenterata (King) as retrogressive so far as the loss of 

 eyes and the closure of the intestine are concerned ; progressive mainly 

 thro.ugh expansion of anterior elements and limitation of pedicle 

 opening to one valve. 



Davidson (11), in his masterly " Summary to the British Fossil 

 Brachiopoda," published in 1884, classified the Brachiopoda in 

 general into fourteen named families and two unnamed ones, containing, 

 respectively, obscure genera with uncertain affinities. In i8gi, 

 Beecher legitimately proposed a new classification, based in a great 

 measure on the nature of the pedicle opening, its hmitations and 

 associated modifications (8). He diAdded the class into four orders. 

 The first and second correspond with the Lyopomata of Owen = 

 Tretenterata, King. The third and fourth with the Arthropomata of 

 Owen ^ Clistenterata, King. 



2R 



